Magnetic sensors used in disk drives or tape drives for reading back magnetically recorded information are usually based on thin film structures utilizing a magnetoresistive effect. In particular, recent sensors are based on a spin dependent scattering phenomenon and are generally called giant magnetoresistive (GMR) sensors or spin valve sensors. These sensors depend on having one magnetic layer, called the free layer, in which the direction of magnetization is free to move in response to the magnetic field applied to the sensor. Another layer is called the pinned layer, in which the direction of magnetization is not free to move and is perpendicular to the direction of the magnetization of the free layer when there is no applied external field. In order to achieve maximum sensitivity and linearity, it is required that the magnetization of the free layer in the absence of an applied field to be substantially parallel with the direction of the recorded track. Accordingly it is required that the magnetization in the pinned layer be substantially perpendicular to the recorded track.
Another requirement for the free layer is that there be longitudinal magnetic bias stabilization. Imposing a preferred magnetization direction in the free layer along the axis of the free layer parallel to the medium and perpendicular to the direction of the track insures good linearity and provides robustness to deleterious effects such as Barkhausen noise.
A common method of providing for pinning the pinned layer is to place a layer of antiferromagetic material, AFM, adjacent to the pinned layer. Then, at some point in the manufacture of the head, the structure is heated above the blocking temperature of the AFM, and an external field is placed on the device which is perpendicular to the recorded track direction. The blocking temperature of an AFM material is the temperature above which the material no longer has any exchange coupling strength. The structure is then cooled in the presence of the field. The applied field will orient the pinned layer in the proper direction and as the AFM cools below the blocking temperature exchange coupling will maintain the orientation of the magnetization in the pinned layer. This is the pinning process.
There are at least two possible techniques to provide for longitudinal biasing of the free layer. A common method is to provide two hard magnets, one on each side of the portion of the free layer which defines the track width. This is referred to as hard biasing. During the manufacture of the sensor, the direction of the magnetization in the hard magnet must be set by placing the sensors in a large magnetic field. The hard bias method has some undesirable attributes such as gradual reduction of sensitivity at track edges and is somewhat difficult to control in manufacturing.
Another method of providing for longitudinal biasing of the free layer is to use an AFM layer to deliver exchange coupling similar to that for the pinned layer. The principle difficulty with this approach is that the direction of magnetization in the free layer must be substantially perpendicular to the direction of magnetization in the pinned layer. Thus the steps of heating and subsequent cooling in a field would be appropriate for one of the AFM layers, but not the other. To solve this problem in the past, two different AFM materials have been used which had distinctly different blocking temperatures. The AFM layer with the highest blocking temperature was set first. Then the field angle was rotated 90° and the second AFM layer was set at a lower temperature. For reasons of magnetic performance and manufacturability there is generally one optimum AFM material which would serve for both pinned and longitudinal stabilization. However because of the requirement to have AFM materials with different blocking temperatures, the optimum choice of AFM materials was compromised.